Convert DartChangeBuilder to use AnalysisSession

pkg/analyzer_plugin/test/support/abstract_context.dart

Index: pkg/analyzer_plugin/test/support/abstract_context.dart
diff --git a/pkg/analyzer_plugin/test/support/abstract_context.dart b/pkg/analyzer_plugin/test/support/abstract_context.dart
index 216b07b13667f320fa81fbfeaeeec8946a4577e2..e124c3309bbf00144b464482ce6be7ba00dc515a 100644
--- a/pkg/analyzer_plugin/test/support/abstract_context.dart
+++ b/pkg/analyzer_plugin/test/support/abstract_context.dart
@@ -4,6 +4,7 @@
 
 import 'dart:async';
 
+import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/visitor.dart';
@@ -61,6 +62,11 @@ class AbstractContextTest {
    */
   bool get enableStrongMode => false;
 
+  /**
+   * Return the analysis session associated with the driver.
+   */
+  AnalysisSession get session => driver.currentSession;
+
   Source addMetaPackageSource() => addPackageSource(
       'meta',
       'meta.dart',